In the manufacture of glass, the constituent materials required to form the glass are raised to a high temperature in a furnace. The materials are combined, in a molten state, and the resultant glass product is, in effect, a super cooled liquid.
In the molten state, the glass temperature is in the range of 1,000.degree. to 1,500.degree. C. and flows fairly freely. It is in this state that it may be formed into bottles, vials, and tubes for use in fluorescent lighting, medical pipettes, surgical instruments, rods or lenses for optical instruments as well as glass sheets or panes for domestic, industrial and automotive applications. Specialist glass may be formed and ground into optical elements for use in telescopes, cameras and other types of photographic equipment.
The present invention is particularly directed to the measurement of thickness of glass, be it in sheet, tube or rod form, as the thickness becomes critical from the view point of meeting a particular specification in terms for example of maintaining safety standards with respect to its application.
In a typical glass works, the molten glass is extruded in a vertical or horizontal line either in sheets, tubes or rods. It is at this stage that measurements are required to be performed on the molten product such as thickness, diameter or wall thickness as the case may be. The range of thickness primarily at issue with regard to the present inventive technique would be in the order of a few microns up to 50 mm, although the invention disclosed herein is not to be taken to be limited to this narrow field of application.
The idea of measuring the thickness of a transparent material by determining the separation between the images or beams reflected from the front and rear surfaces of the material as the result of the impingement of a single beam of light directed at an angle of incidence onto the front face, wherein the separation is a measure of that thickness, is already known.
In particular a thickness measuring apparatus designed for this purpose is disclosed in U.S. Pat. No. 3807870.
This prior system employs the use of an array of closely spaced light sensors which are arranged to intercept the reflected beams, the distance between selective ones of the sensors receiving the beams being taken as a measure of the thickness of the material.
The light sensors, preferably in the form of photodiodes, are of extremely small dimension and produce electrical signals in accordance with the amount of light they receive from each reflected beam.
A scanning detector is employed electronically to scan the array and a trigger voltage is produced each time the signal exceeds a given threshold value. Thus a pair of trigger signals are produced during each scan and the time interval between the signals is used to determine the thickness of the workpiece.
This prior system has not proved entirely successful mainly because of its inability to provide the accuracy of measurement required in many applications in the field.